U.S. Pat. Nos. 6,468,811 and 6,387,707 (incorporated herein by reference) disclose a system providing for a novel type of electrokinetic transport of particles suspended in a polarizable liquid medium. The particles, suspended in the medium, are placed into the gap between two essentially planar electrodes, which lie in different planes. An AC voltage (rather than a DC voltage, as in electrophoresis) is applied across the fluid gap between the two electrodes in contact with the electrolyte solution to induce fluid flow and particle transport. Near the active electrode—defined as that connected to an external AC (and optionally superimposed DC voltage), with the counterelectrode grounded—fluid flow and particle transport are directed along the induced electric field, i.e., parallel to the electrode surface. The mode of transport is not electrophoresis or dielectrophoresis.
By way of either illuminating the active planar electrode composed of suitable material or, alternatively, by selectively chemically patterning the electrode, regions of higher and lower impedance can be defined on the electrode, for example by reducing the impedance in the illuminated or patterned portion(s) of the electrode, and, in the presence of a low frequency applied AC electric field, induce movement of particles into the illuminated (or patterned) areas. See U.S. Pat. No. 6,468,811. Patterning can be accomplished using chemical or other means, for example in order to produce dielectric layers of differing thickness on the active electrode. Selective patterning by optical or chemical means provide the means to define a variety of configurations of particle assembled on the electrode surface in accordance with the patterns, move particles in or out of these configurations, or, in the case of optical patterning, reconfigure these configurations as desired, thereby providing the ability to assemble or disassemble and to reconfigure arrays of particles. International Application Publication No. WO/0120593, entitled “SYSTEM AND METHOD FOR PROGRAMMABLE ILLUMINATION PATTERN GENERATION” discloses a system for programmable control of the electrokinetic movement of particles described above. In this system, electric field-induced assembly of planar particle arrays at an interface between an electrode and an electrolyte solution is again employed, as is the modulation of the impedance of the electrolyte-insulator-semiconductor structure by UV light. In addition, this system employs a control system and user interface to permit the real-time, interactive control over the electrode areas illuminated, to thereby generate, selectively, areas of modified impedance. Spatially and temporally varying the illumination pattern on the lower electrode provides real time control over the movement of suspended particles. International Application Publication No. WO 02076585, entitled “ANALYSIS AND FRACTIONATION OF PARTICLES NEAR SURFACES,” discloses a system to further fine-tune movement of suspended particles. In this system, a mixture of particles can be fractionated according to their relaxation frequencies, which in turn reflect differences in size, surface composition or other properties of the particles. Particles with relaxation frequencies greater than that of the applied voltage will be separated from others in the mixture and transported by electrohydrodynamic forces generated in response to the applied electric field.
In all of the foregoing systems, particle movement is used to assemble and disassemble arrays related to a variety of bioanalytical assays. In particular, the inventors discuss assembly and disassembly of arrays of beads bound to oligonucleotides or other biological materials, and arrays of cells. The ability to form, assemble, hold, separate and disassemble arrays selectively, as well as to separate particular beads from the arrays, provides a number of advantages in assay procedures, as more fully explained in the patents and references set forth above.
When using illumination to vary the impedance, one must apply a relatively large voltage (typically about 6 Vpp) across the electrodes to induce a sufficiently large ionic flow to move the particles. When using such voltages, depending on the doping concentration and the illumination level, the semiconductor surface may be forced into strong accumulation or inversion thereby forming conduction channels on the surface, which in turn facilitate lateral drifting of the photo-generated carriers residing in the illuminated region. As a result, the electric field gradient at the edges of the illuminated regions is lowered, making the edges of the pattern of particles one is seeking to form by illumination “blurred” and less distinct. Stronger illumination was observed to enhance the ionic flow. However, the level of illumination has an upper limit, over which the effective regions for particle assembly are widened and the edges become less distinct. A lightly doped surface layer may have the advantage of enhanced carrier generation due to its reduced density of carrier recombination sites. However, this also has a disadvantage, in that, with the depth of the surface layer transiently exceeding the narrowed surface depletion width under positive bias, the lateral diffusion of photo-generated carriers can be enhanced in the substrate which can be collected by the space charge region in the surrounding dark regions at surface, thereby effectively lowering the surface-voltage contrast between the illuminated and the dark regions. Again, as with the effect of large voltages described above, this will effectively widen the regions of lowered impedance about the illuminated regions and make the voltage difference and transition edge between the light and dark regions less distinct. Consequently, the magnitude of induced ionic flow and low-frequency dielectrophoretic forces under the influence of the gradient ∇(E.E) acting upon the particles are limited. The minority carrier recombination lifetime is an inherent property of the material, and cannot be substantially altered by illumination at levels of interest.
Thus, a system in which there is direct (or improved) control over the spatial location and resolution of the voltage drop to provide a more defined and configurable electric field gradient would be advantageous. In addition, if the effect of interfacial impedance modulation is achieved by an active voltage applied directly at the electrolyte/insulator interface, voltages applied across the fluid gap generally may be lower than those required to generate the same field gradient when using illumination (as described in U.S. Pat. Nos. 6,468,811 and 6,387,707). In the illumination-based systems described above, the AC voltage is applied across the entirety of the planar upper and lower electrodes, and one can only apply a single frequency to the entire structure at any point in time. In contrast, if the frequency of the AC potential is spatially programmable in different locations across the substrate surface, particles or other materials can be separated into different areas of the substrate based on their relaxation frequencies. See, e.g., International Application Publication Nos. 01/20593 and 02/076585.
While illumination provides a flexible and adaptable method to achieve dynamic reconfigurability and hence optically programmable fluid flow and particle transport, this method readily applies only to those regions of an electrode permitting optical access. In certain configurations and applications, it will be desirable to exert control over a larger portion of an electrode than that accessible optically. It thus will be desirable to provide methods and compositions permitting dynamic reconfigurability and programmable transport and assembly of fluids and particles in a manner not requiring illumination.
One possible system to directly generate electric field gradients would be an array of microelectrodes capable of generating desired configurations of electric fields on activation, arrayed on a substrate and underlying the particle/electrolyte suspension. Selective activation could be used to control particle movement in the same manner as accomplished by UV illumination or chemical patterning of the insulating layer. One problem would be providing a chemically and electrically uniform array surface, which would not itself generate spatial modulation in the electrolyte other than those desired. In addition, construction would be difficult as each electrode would need to be individually connected to a control circuit or multiplexed.
Thus, what is needed is a practical and effective system to provide the same degree of spatial control over electric field generation as can be achieved with known systems involving illumination of a substrate surface or surface patterning, preferably in the form of an integrated microelectronic device permitting inexpensive mass production.